Apparatus and method for extrusion of thin-walled tubes

ABSTRACT

The present invention provides an apparatus and method for manufacturing polymeric thin-walled tubular members, which are well-suited for use as vascular grafts. The apparatus of the present invention enables extrusion of a tubular member having an extremely thin wall thickness so as to facilitate passage through tortuous vascular passageways. The apparatus achieves uniform wall thickness in a tubular member by establishing and maintaining axial alignment of a ram having a concentrically disposed guide rod therein with a die supported in concentric relation to the guide rod.

FIELD OF THE INVENTION

[0001] The present invention relates generally to an apparatus andmethod for manufacturing a thin-walled tubular member, well-suited toform an implantable prosthesis that is used to repair and/or replace orotherwise treat a vascular defect. More particularly, the presentinvention relates to an extrusion apparatus having a concentricallysupported die for extrusion of extremely thin-walled tubular members,such apparatus employing an extrusion method that avoids deformation,warping, disfigurement, asymmetry and other distortions of the tubularmember produced thereby.

BACKGROUND OF THE INVENTION

[0002] Extrusion encompasses a plurality of well-known processes thatfeature low tooling and labor costs, making extrusion a desirablemachining process especially for tubular profiles. During an extrusionprocess, a solid polymer (i.e., pellets, chips, beads, powder and thelike) is generally fed through a transport section into a rotating screwor extrusion pipe via a feeder or hopper. Plastic is slowly heated as itis pressed forward toward a die, becoming a homogeneous melt that issubsequently forced through the die to form a continuous-length shape.Once cooled, the extrusion is cut and shaped to a desired length andconfiguration.

[0003] Ram extrusion is a specific extrusion method that utilizespressure sintering for the continuous production of profiles fromhigh-molecular-weight polymers. A ram forces a pre-formed mass ofpolymeric material, such as pre-compressed powder, into a die (typicallyheated) having the requisite cross-section. The individual particles ofthe mass sinter together under the effects of heat and pressure togenerate an extrudate.

[0004] Achievement of a desirably thin tube is impaired due to thelimitations of conventional extrusion processes. It is desirable tomaintain uniform wall thickness during an extrusion process and providegenerous radii that improve the flow of the composition through the dieand reduce stresses associated therewith, thereby significantly reducingthe cost and improving the consistency of the product. For instance,maintaining the uniformity of the wall thickness helps to maintain themaximum running speed of the extruder and the straightness of the part.Because extruded parts are only contained by the metal at the die, theytend to move or sag downline to the detriment of product uniformity.Although it is relatively easy to manipulate the physical and mechanicalproperties of the material to match performance specifications, itremains nonetheless difficult to manipulate thin-walled members due tothe flexible nature of the resultant extruded product.

[0005] Additionally, a common problem encountered during extrusion isdeviation of tubular thickness and center deviation during the extrusionprocess. In a ram extrusion apparatus, the die not only shapes the finalextrudate, but it also determines the quality of the extrudate by thecombined effects of pressure and temperature. Generally, extrusion diesfor hollow profiles consist of an extrusion pipe, a mandrel and a ram.In the extrusion of hollow profiles such as tubes, dense powder mustslide over two surfaces (i.e., those of the extrusion pipe and themandrel), thereby inducing escalated pressure during the extrusion ofprior art tubes (having relatively thin walls in the range of 2-3 mmthickness) and counter pressure due to friction. Extrusion pressures arethus higher for such relatively thin prior art profiles than thoseexperienced for solid profile extrusion or tubular profiles withincreased wall thickness. As a result, when extruding materials, thereis an inherent risk of exceeding the upper pressure limit of the powderand getting tablet formation, especially during cooling when theextrudate shrinks onto the mandrel. In addition, the critical componentsof extrusion equipment (e.g., mandrel; extrusion pipe; die) aresubjected to high forces and pressure which cause relative movementbetween the components. Such relative movement typically results in someoff-center, eccentric alignment which is acceptable when extrudingrelatively thick-walled tubular members. However, with thin-walledstructures, the eccentric off-set may cause insufficient material to bedirected to a portion or portions of the resulting extrudate.Accordingly, wall portions may result which are unacceptably too thinand/or include discontinuities. Thus, continuous maintenance of thealignment of key extruding elements is critical in forming thin-walledmembers.

[0006] A beneficial material that is widely used in extrusionapplications, including ram extrusion, is polytetrafluoroethylene(PTFE). PTFE undergoes cold flow so as to change dimension when pressureis applied, even at room temperature. PTFE can be extruded from a resinpowder mixed with a liquid lubricant or from a conventional paste. Whenproperly processed, PTFE exhibits superior properties inherent influoropolymer resins, such as non-aging characteristics, chemicalinertness, heat resistance, toughness and flexibility, low coefficientof friction, negligible moisture absorption, improved resistance todeformation under load and minimal sagging when in billet form. BeyondPTFE, most polymeric materials are suitable for extrusion, including butnot limited to high density and low density polyethylene, rigid andflexible polyvinylchloride, polypropylene, polyurethane, thermoplasticrubber, acrylic, cellulose acetate butyrate, polycarbonate,polyethersulfone and polyphenylene sulfide.

[0007] One technique for forming PTFE tubing is to use a wet flow pasteextrusion process. The paste extrusion process yields a “green tube”that is then subjected to secondary operations such as heating,expansion and sintering to yield an expanded PTFE (ePTFE) tube having aporous node and fibril structure. The green tube produced by the priorart paste extrusion process has a relatively thick wall and a smallinternal diameter, and the resultant ePTFE tube exhibits a relativelythick wall thickness. It is desired to obtain structurally-soundthin-walled tubes having thinner walls than that found in the prior art.It is especially desired to obtain thin-walled PTFE green tubes andePTFE tubes which can be used in medical applications, such as withgrafts and stent/grafts.

[0008] It is therefore desirable to provide a mechanism and associatedmethod for repeatedly and predictably producing an elongate tubularmember having a very thin wall thickness. Such a member is conducive toemployment in a variety of applications wherein prolonged patency of thetube complements inherent circumferential strength without compromisinga need for minimal wall thickness.

SUMMARY OF THE INVENTION

[0009] It is an advantage of the present invention to manufacturethin-walled tubular members in an inexpensive and easily executedextrusion process.

[0010] It is a further advantage of the present invention to extrudetubular elements having a very thin circumferential wall of uniformthickness.

[0011] It is still a further advantage of the present invention toextrude such thin-walled tubular members while preserving the structuralintegrity of the member.

[0012] It is yet another advantage of the present invention to execute aprocess for the extrusion of thin-walled tubular members via anapparatus having concentrically supported extruder elements.

[0013] In the efficient attainment of these and other advantages, thepresent invention provides an extrusion apparatus and method formanufacturing a generally tubular profile with a thin-walled structure.The apparatus is well-suited to extrude polymeric compositions, such asPTFE in forming prostheses.

[0014] The extrusion apparatus includes an assembly having a pluralityof successively spaced support platens interconnected by a plurality oftruss members. The truss members maintain the platens in a parallelarrangement. Each support plate includes a central aperture therethroughsuch that alignment of the plates effects axial alignment of theircentral apertures.

[0015] A generally cylindrical guide rod is threaded through the supportplates in concentric relation with a ram supported therearound. Such ramincludes a piston at an extent thereof that is disposed in a bore of abarrel. An annular wiper element is also mounted onto the ram andpositioned adjacent the piston so as to ensure additional concentricsupport of the guide rod in relation to the ram and further in relationto the barrel. The guide rod extends the length of the barrel to a frontextruder plate at which a die is coupled to the barrel by a die-mountingcollar positioned annularly therearound. A mandrel is formed at a tipportion of the guide rod, the mandrel extending at least partially intothe orifice of the die. Advantageously, the guide rod, the barrel andthe ram are supported concentrically such that these elements share acommon longitudinal axis with the central apertures of the supportplates. The support is maintained so that movement of the guide rodrelative to other elements is generally constrained and a relativelyconstant annular gap is maintained between the guide rod and the barrel.

[0016] Preferably, the extrudate passageway tapers into the desiredouter diameter of the die, wherein the outer diameter dimension ismaintained. In addition, it is preferred that the mandrel be flexiblyformed, or be flexibly connected to the guide rod so as to be movablerelative thereto. As such, the mandrel has the ability to adjust duringan extrusion process and self-center. With the ram forcing the pre-formthrough the die, and about the mandrel, the mandrel is able toself-center in response to the pressure of the pre-form beingrelatively-evenly applied circumferentially about the mandrel. Theself-centering effect ensures proper alignment of the mandrel relativeto the die, which is especially critical in forming extremelythin-walled tubing.

[0017] In operation, a pre-form billet of selected polymeric material isplaced within the barrel at the front end thereof. Steady movement ofthe ram effects corresponding movement of the piston, thereby inducingpressure and heat on the billet as the piston moves through the borearound the guide rod. The resultant pressure and heat cause the billetto become a flowable extrudate that traverses a circumferentialperiphery of the guide rod and mandrel for delivery of a resultantthin-walled tubular profile through an exit opening in the die. Becauseof the inventive arrangement, the extruded tube is formed with uniformwall thickness. The method and apparatus of the subject inventionovercome deficiencies of the prior art and allow for extremelythin-walled tubular structures to be uniformly extruded.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIGS. 1 and 2 show top and side views of an extrusion apparatus ofthe present invention having concentrically supported extruder elementsdisposed therewithin.

[0019]FIG. 3 shows a perspective view of an extrusion mold assembly usedin connection with the subject invention.

[0020]FIG. 4 shows an enlarged view of a die and mandrel used inconnection with the subject invention.

[0021]FIG. 4A shows an enlarged cross-sectional view of a die andmandrel used in connection with the subject invention.

[0022]FIG. 5 shows a side view of a truss member used in connection withthe subject invention.

[0023] FIGS. 6(a) and 6(b) show cross sections of the guide rod andbarrel with an annular gap being formed therebetween.

[0024]FIG. 7 shows a cross section of a tubular prosthesis incorporatingextruded tubular members produced in accordance with the subjectinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] The present invention provides an apparatus and method formanufacturing polymeric tubular members, which are well-suited for useas vascular grafts. The apparatus of the present invention enablesextrusion of a tubular member having an extremely thin-wall thickness soas to facilitate passage through tortuous vascular passageways. Theapparatus achieves uniform wall thickness in a tubular member byestablishing and maintaining axial alignment of a ram having aconcentrically disposed guide rod therein with a die supported inconcentric relation to the guide rod.

[0026] Now referring to the figures, wherein like elements areidentically numbered, the configuration and operation of the presentinvention can be described.

[0027] Specifically referring to FIGS. 1 and 2, an extruder 10 of thepresent invention is provided for the extrusion of a pre-form (i.e.,billet) of polymeric material into a thin-walled tubular member. Thechoice of raw material is an important factor affecting the finalextruded product, and there are certain requirements of the raw materialthat should be fulfilled so as to optimize extrusion. Such requirementsinclude, but are not limited to, good free flowability, high packingdensity and granule stability and hardness that is suitable for theapplication. Desirable materials that meet such requirements includehigh molecular weight polymers such as fluoropolymers, particularly,polytetrafluoroethylene (PTFE), that feature low water absorption,reduced friction and easy sterilization. Although reference is madeherein to PTFE, it is understood that any polymeric composition may beincorporated that is compatible with operation of the present invention.

[0028] Extruder 10 includes an extrusion mold assembly 12 (separatelyillustrated in FIG. 3) that provides a scaffolding for effectiveconcentric support of requisite extruder elements thereby. Assembly 12,having a forward end 12 a and a rearward end 12 b, generally includes aseries of platens including stationary platens 14 and 18 and traverseplaten 16. Platens 14 and 18 are secured to one another in spacedalignment by a plurality of primary truss members 19.

[0029] As shown in the Figures, four of the truss members 19 areutilized, although other quantities may be used. It is preferred thatthe platens 14, 18 be generally rectangular and that the truss members19 be secured in orifices 14 a, 18 a respectively, in proximity to thecorners of the platens 14, 18. Preferably, the truss members 19 arecountersunk into the platens 14, 18, as more fully described below. Withthis arrangement, strong perimetric support for the platens 14, 18 maybe provided which will maintain the platens 14, 18 in a desired parallelarrangement.

[0030] The traverse platen 16 is formed with orifices 16 a and isdisposed in sliding engagement on central body portions 19 a of thetruss members 19 with the respective central body portions 19 a passingthrough the orifices 16 a. As is known to those skilled in the art,slide bearings B may be mounted in the orifices 16 a to aid in slidingmovement. It is critical to maintain parallelism between the traverseplaten 16 and the platens 14, 18. Thus, it is desired that the slidebearings B be provided with relatively small clearances relative to thetruss members 19, thus, minimizing twisting of the platen 16 duringmovement and maintaining parallelism.

[0031] Each of platens 14, 16 and 18 further includes a central aperture14 b, 16 b, and 18 b, respectively, therein to accommodate passage ofextruder elements therethrough. Platens 14, 16 and 18 are notablycoupled in parallel alignment so as to ensure corresponding parallelalignment therebetween for maintaining concentricity of the extruderelements throughout an extrusion cycle.

[0032] A front extruder plate 20 is also provided that similarlyincludes a plurality of orifices 20 a defined therethrough toaccommodate insertion of corresponding secondary truss members 23. Plate20 also includes a central aperture 20 b that accommodates passage ofextruder members, as further described hereinbelow. Secondary trussmembers 23 are similarly formed to the truss members 19 and are securedto the plate 20 and the platen 16 through countersunk connections tomaintain a parallel arrangement therebetween, also as described below.Truss members 23 not only provide support for plate 20, but also ensureparallel arrangement of the plate 20 and the platens 14, 16, 18 andmaintain coaxial alignment of apertures 14 b, 16 b, 18 b and 20 b.

[0033] Now referring particularly to FIGS. 1 and 2, a preferredconfiguration of concentric extruder elements may be described. Aperture14 b of plate 14 insertably accommodates a free extent of a guide rod 26therethrough. Guide rod 26 is an elongate, generally cylindrical membertraversing a length of assembly 12 in concentric relation with orifice14 b. Guide rod 26 is at least partially retained in such relation byguide rod bushing 28 that is annularly disposed in orifice 16 b oftraverse platen 16. Guide rod 26, is preferably hollow, but may beformed solid.

[0034] Guide rod 26 linearly extends in concentric relation throughorifice 16 b so as to be coaxially disposed within tubular ram 30 andmore particularly in a bore 31 defined along a longitudinal axisthereof. Ram 30, along with guide rod 26, extend in coaxial relationthrough an elongate barrel 34 having a bore 36 defined therewithin. Rambushing 32, that is positioned adjacent rear surface 18 c of plate 18,circumscribes a periphery of ram 30 so as to maintain the coaxialrelation of ram 30 relative to the barrel 34.

[0035] Ram 30 is coupled with piston 38 that is also disposed insidebore 36. An annular wiper element 39 is mounted onto the ram 30 anddisposed adjacent to piston 38 so as to provide additional concentricsupport of the guide rod 26 in relation to the ram 30 and further inrelation to the bore 36 during an extrusion operation. The wiper element39 acts as a slide bearing between the barrel 34 and the guide rod 26 inproviding support for the guide rod 26. There is tight contact betweenthe guide rod 26, the wiper element 39, and the barrel 34.

[0036] As shown in FIGS. 1 and 2 and more particularly illustrated inthe enlarged view of FIG. 4, a die mounting collar 40 having an opening40 a defined therethrough is annularly disposed about each of die 42 anda front extent of barrel 34 such that collar 40 concentrically supportsthe die 42 relative to the barrel 34. In particular, the die mountingcollar 40 includes stepped portions 40A, 40B, 40C ofincreasingly-reduced diameters. The stepped portion 40A is formed toreceive the front extent of the barrel 34 so that the die mountingcollar 40 is concentrically aligned therewith. The stepped portion 40Bis formed to receive enlarged end 42A of the die 42 with the steppedportion 40C being formed to allow the passage therethrough of theextrusion end 42B of the die 42. The fixed relationship of the steppedportions 40A, 40B, 40C allows for the die 42 to be concentricallyaligned with the barrel 34 and provides holding force to minimizerelative movement between the die 42 and the barrel 34. As a furtherenhancement, a positioning collar 80 may extend from the enlarged end42A which is formed to coact with a positioning seat 82 formed in thefront extent of the barrel 34. The stepped portions 40A, 40B, 40C andthe coaction of the positioning collar 80 and the positioning seat 82not only further provide for proper alignment between the die 42 and thebarrel 34, but also provide additional holding force to maintain properalignment of the die 42 and the barrel 34. To allow for placement of apre-form into barrel 34, the die mounting collar 40 is removably mountedto the plate 20, preferably, using bolts.

[0037] With reference to FIG. 4A, die 42 includes a tapered extrudatepassage 43 therethrough that terminates in a cylindrical extrudate exitopening 45. The parameters of the exit opening 45 are dictated by thedesired dimensions of the extrudate to be formed and are partiallydefined by mandrel 48 about which the polymeric extrudate flows uponcompression applied by the ram 30.

[0038] A tip portion of the guide rod 26 may define the mandrel 48,wherein the mandrel 48 is unitarily formed with the guide rod 26; or,more preferably, the mandrel 48 is formed to be removable from the guiderod 26. It may be desired to allow for easy removal of the mandrel 48,to allow for maintenance and/or easy loading of a billet pre-form ofpolymeric material. It is preferred that the mandrel 48 have aself-centering characteristic relative to the die 42.

[0039] In a preferred embodiment, as shown in FIG. 4A, the mandrel 48 ispreferably formed from multiple parts 48A, 48B which are flexiblyconnected so as to allow relative movement therebetween. By way ofnon-limiting example, the first part 48A of the mandrel 48 is generallycylindrical having a ball-shaped joint member 49A formed at an endthereof. The second part 48B is formed with a socket 49B shaped toreceive the ball-shaped joint member 49A in a ball-socket jointarrangement. Although the second part 48B has a tapered portion 48Cwhich extends beyond the socket 49B thereby inhibiting the ability ofthe first part 48A to move relative to the second part 48B, the flexibleconnection of the ball-socket joint permits relative movement betweenthe first and second parts 48A, 48B. The tapered portion 48C provides asmooth transition surface between the bore 36 and the exit opening 45 inpartially defining the extrudate passage 43. It is therefore preferredthat all surface direction changes found on the tapered portion 48C bedefined by rounded or gradual transitions to avoid sharp edges orcorners. Likewise, transitions in the passage 43 are to be gradual orrounded to avoid sharp edges or corners.

[0040] It is preferred that the second part 48B be rigidly mountable tothe guide rod 26 (not shown in FIG. 4A). Accordingly, the first part 48Amay move relative to not only the second part 48B, but also the guiderod 26. To facilitate mounting, a threaded stem 70 is formed tothreadedly engage threads found on an internal surface of the guide rod26. It is desired to achieve a smooth transition between the mandrel 48and the guide rod 26 without any change in diameter. The threaded stem70 may be unitarily formed with the mandrel 48, or, as shown in FIG. 4A,is threadedly received in a socket 72 in the mandrel 48.

[0041] During use, the mandrel 48 is responsive to the extrudate so asto be self-centering relative to the die 42. With the extrudate beingslowly forced through the die 42 from a pre-form, the polymeric materialis fed circumferentially about the first part 48A relatively evenly.Because of the steady and even feeding of material, the polymericmaterial urges the first part 48A into concentric alignment relative tothe die 42. It should be noted that anomalies (e.g., lumps; bubbles) maybe in the extrudate which is are urged past the first part 48A, thuscausing misalignment thereof. Although an anomaly will cause a possibledeformity in the resulting extrudate (i.e., irregularity in wallthickness), the mandrel 48 will self-center after the anomaly passesthrough. Because of this self-centering featuring, extremely thin-walledtubes, having wall thicknesses of about 250 μm and less, can be formed.Prior art rigidly-fixed mandrels limit the thickness of the extrudate.Any off-center deviation with a rigidly-fixed mandrel results in aconsistently improperly formed tube because of the rigid fixing (e.g.,eccentric wall formation). Furthermore, with the repeated loading ofpre-forms into a ram extruder, a self-centering feature of the mandrel48 is highly desirable to correct for any misalignment in mandrelmounting.

[0042] As an alternative, the mandrel 48 can be unitary and mounted ontothe guide rod 26 via a flexible connection so as to be movable relativethereto, as shown schematically in FIG. 4. For example, the mandrel 48may be partially threaded onto the guide rod 26 via the stem 70, therebyallowing relative movement between the mandrel 48 and the guide rod 26.Any form of flexible connection known to those skilled in the art may beutilized (e.g., an elastic joint; a bellows connector).

[0043] Referring again to FIG. 1, to further enhance secure support ofthe plate 20, tertiary truss members 33 may be mounted in proximity tothe barrel 34. The tertiary truss members 33 are preferably countersunkinto the plate 20 and the platen 14 in similar fashion as the trussmembers 19 described below.

[0044] Piston cylinders 35 are mounted to and extend between the frontplate 20 and the platen 14 with drive pistons 37 being slidably disposedwithin the piston cylinders 35. Stems 39 of the drive pistons 37 extendfrom the cylinders 35 and through the platen 18 via orifices 18 d. Thestems 39 are sealed within the orifices 18 d to prevent leakage from thecylinders 35. Ends 41 of the stems 39 are countersunk into the traverseplaten 16 in similar fashion to the securement of the truss members 19described below. Beneficially, the cylinders 35 add additional rigidityto the assembly 12, along with the countersunk connections with thetraverse platen 16.

[0045] The cylinders 35 are charged with any driving medium known tothose skilled in the art, such as hydraulic fluid or air. It ispreferred that hydraulic fluid be used and that the fluid be steadilyand slowly charged into the cylinders 35. As such the ram 30, via thetraverse platen 16, can be steadily driven towards the die 42.

[0046] In operation, a ram extrusion process is initiated in extruder 10by placing a billet or pre-form of a polymeric composition (not shown)into the bore 36 of the barrel 34. To facilitate placement of thebillet, it is preferred that the die 42 be formed to be removed from thefront plate 20. The billet can then be inserted into the barrel 34.Driving motion (i.e., via hydraulic action of drive pistons 37)transmitted to plate 16 advances plate 16 along truss members 19 towardplate 18, thereby advancing ram 30 through barrel 34. Advancement of ram30 inures translational movement to piston 38 and wiper element 39 alongguide rod 26, maintaining guide rod 26 in concentric alignment with ram30, mandrel 48, extrudate passage 43 and exit opening 45. Furtheradvancement of ram 30 exerts pressure upon the polymeric billet untilthe billet becomes a flowable extrudate. The extrudate flows throughextrudate passage 43 and exit opening 45 along a smooth outer peripheryof mandrel 48 to define a tubular profile thereby.

[0047] As indicated, various connections in the extruder 10 are achievedwith countersinking. To describe the countersinking connections,reference will be made to connections between the truss members 19 andthe platens 14 and 18. It is to be understood that the other countersunkconnections (i.e., the secondary truss members 23, the tertiary trussmembers 33, the piston stems 39) are formed dimensionally andstructurally in the same fashion. With reference to FIG. 5, each of thetruss members 19 is formed with the central body portion 19 a, having afirst diameter D₁, and end portions 22, each having a second diameter D₂smaller than the diameter D₁. As a result of this structure, annularfaces 25 are defined at the ends of the central body portion 19 a whichface outwardly. Preferably, the faces 25 are generally flat andperpendicularly disposed to a central longitudinal axis CA of the trussmember 19. Correspondingly, the orifices 14 a, 18 a are each formed witha first diameter portion 27, an annular shoulder 29, and a reduceddiameter portion 31. The first diameter portion 27 is formed with adiameter D₃ which is equal to the diameter D₁ plus a minimal clearanceδ, while the reduced diameter portion 27 is formed with a diameter D₄which is equal to the diameter D₂ plus a minimal clearance δ′. Theclearances δ, δ′ are set to allow for assembly of the elements, but itis preferred to keep such clearances to a minimum, especially theclearance δ. For example, with the diameter D₁ being equal to 2.0 inches(+0.0005/−0.0000 inches), the clearance δ is 0.001 inches(+0.0005/−0.000 inches) and thus the diameter D₃ is nominally 2.002inches; whereas, with the diameter D₂ being equal to 1.75 inches(+0.01/−0.01), the clearance δ′ is 0.05 inches (+0.005/−0.005), and thusthe diameter D₄ is nominally 1.85 inches. The tendency of the trussmembers 19 to shift will be a function of the clearances δ, δ′, and muchmore a function of the clearance δ′. In addition, the extent the centralbody portion 21 extends into the first diameter portion 27 not onlydictates the extent to which the truss members 19 can shift, but, also,dictates the extent to which the truss members 19 are prevented frombuckling (i.e., cause lateral deflection of the central body portion 19a). It is preferred that the central body portion 19 a extend a length linto the platen with the length l being at least 33.3% of the thicknessT of the platen (e. g., l is at least 1 inch, where T is 3 inches). Allof the countersunk connections are formed with the same dimensionalrelationships. It is further preferred that the shoulder 29 be generallyflat and disposed perpendicularly to a central axis CA of the respectiveorifice 14 a, 18 a.

[0048] In securing the truss members 19 to the platens 14, 18, theannular faces 25 are pressed into engagement with the shoulders 29, withthe first diameter portions 27 partially accommodating the central bodyportions 19 a and the end portions 22 extending through, and extendingfrom, the reduced diameter portions 31. The bearing engagement of thetruss members 19 with the orifices 14 a, 18 a, especially at the face25/shoulder 29 interface and at the central body portion 21/firstdiameter portion 27 interface, constrains relative movement between theparts, including possible buckling of the truss members 19. Tofacilitate securing the truss members 19 to the platens 14, 18, the endportions 22 are at least partially threaded and nuts N are secured ontothe end portions 22. Any securing means known to those skilled in theart may be utilized to secure the truss members 19 to the platens 14,18.

[0049] It is further preferred that the platens 14, 16, 18 and the plate20 be each formed of sufficient thickness so as to not deflect, twist,or shift relative to the truss members 19. As an exemplary embodiment,with the platens or plate being made of AISI 1060 steel, a thickness Tof 3 inches may be provided.

[0050] With the subject invention, extremely thin-walled tubularstructures may be achieved desirably having a wall thickness less thanor equal to about 250 μm and being longitudinally expandable to a valuegreater than or equal to 1000%. The subject invention is able to achieveextruded thin-walled members which are not fully self-supporting (in acooled state). Because of the rigid securement of the plate 20 alongwith the platens 14, 16, 18, movement of the guide rod 26 is constrainedduring extrusion, thereby allowing the mandrel 48 to accuratelyself-center relative to the die 42, with the resulting extrudate beingconsistently formed uniformly (i.e., uniform wall thickness), even atsuch small wall thicknesses of less than, or about, 250 μm.Specifically, the guide rod 26 is constrainedly held at various pointsalong its axial length, including by the platen 14, the traverse platen16, and the wiper element 39 to maintain its position. As a result asshown in FIGS. 6(a) and 6(b), an annular gap 100 defined between theguide rod 26 and the barrel 34 is constantly maintained with the samedimensions. Consequently, the mandrel 48 is concentrically alignedrelative to the die 42. With the guide rod 26 being constrainedly heldin a property-aligned position, the mandrel 48 has a fixed referencepoint relative to which the self-centering function acts. As shown indashed lines in FIGS. 6(a) and 6(b), if any of the truss members (19;23; 33) were allowed to shift relative to the plate/platens and/orallowed to buckle, there will be relative movement between the guide rod26 and the barrel 34, thereby affecting the dimensions of the annulargap 100. The subject invention greatly reduces and/or eliminates suchrelative movement.

[0051] It is important to note that the guide rod 26 need not becentrally located relative to the barrel 34. For example, an extrudatemay be required with a thickened wall portion (i.e., an eccentricarrangement). The subject invention may also be utilized in thisapplication. Advantageously, the extruder 10 allows for rigid securementof the key elements in allowing uniform extrusion—the holdingarrangement need not be centrally held relative one element to another.With such an arrangement, the self-centering feature of the mandrel 48need not be used.

[0052] With certain polymeric materials, e.g., PTFE, the tubularextrudate collapses upon exiting the opening 45 when extruded atthicknesses of about 250 μm or less. To overcome this problem, the tip48D of the mandrel 48 is formed open and in communication with an openpassage 48E extending through the mandrel 48. The passage 48E is definedthrough all parts of the mandrel 48 (e.g., the first part 48B; thesecond part 48C) and any connection members (e.g., the threaded stem70). A stable medium is ejected from the tip 48D into the lumen of theextrudate to pressurize the lumen, and thus provide support. Preferably,the guide rod 26 is hollow and extends through the platen 18 allowingfor a source of compressed air (80-100 p.s.i.g.) to be coupled thereto.The hollow lumen of the guide rod 26 is placed into communication withthe passage 48E, and the compressed air is urged the length of the guiderod 26 and through the mandrel 48.

[0053] As further enhancements, the barrel 34 and/or the die 42 may beheated using conventional technology, such as, for example,resistance-wire heating elements.

[0054] Extrusion apparatus 10 of the present invention is desirablywell-suited to manufacture vascular prostheses having extremely thinwalls, an example of which is shown in FIG. 7. As is evident from FIG.7, prosthesis 50 is a tubular vascular conduit having each of anextruded luminal sheath 52 and an extruded PTFE exterior sheath 54derived from extruder 10. Each sheath 52, 54 desirably has a thicknessof less than or equal to about 200 μm (the sheaths 52, 54 are preferablyformed from expanded PTFE (ePTFE) which is processed from unexpanded“green” tubes extruded by the extruder 10 of the subject invention).

[0055] An illustrative example of the operation of the present inventiveextrusion apparatus and method is provided in the example below. It isemphasized that the values provided herein merely constitute an exampleof how the present inventive process can be completed, and that thesevalues can be affected in view of material selection, desired tubethickness, piston velocity and related factors as described herein. Itis therefore understood that any or all steps may be modified insequence or duration to adapt to different applications.

EXAMPLE

[0056] With reference to extruding PTFE, ICI CD 123 resin is blendedwith isopar G lubricant at a ratio of 15.5. to 18% isopar by weight. Theresin and lubricant are blended in a V-blender for approximately 15minutes to form a paste thereby. A pre-former that compresses the pasteinto pre-form (i.e., cylindrical billets) is loaded with approximately100 g of paste per pre-form. The paste is compressed into cylindricalpre-forms at hydraulic pressures ranging from 100 to 500 psi. Heat maybe applied during compression in the range from about room temperatureto about 125° F. It is preferred that the pre-forms be formed radiallysmaller than the bore 36 of the barrel 34. It is desired to obtain aclearance of 0.03125″ (as measured on diameter) between the preform andthe barrel 34.

[0057] After removal of the pre-forms from the pre-former, the pre-formsare loaded into the extruder 10 and extruded as described above, formingan axially oriented green PTFE tube with wall thickness ranging fromabout 100 to 250 microns. The hydraulic pressures applied duringextrusion range between 200 and 500 psi. The pressure applied by thepiston 38 to the pre-forms is in the range of about 800 to 900 p.s.i.g.The desirable reduction ratio (i.e., ratio of pre-form cross-sectionalarea to cross-sectional area of extruded tube) is in the range of 125 to350. During the extrusion, the extruder die and barrel are heated fromabout 77° F. to about 125° F. As the extrudate exits the extruder die,air at 80-100 p.s.i.g. is injected into the lumen of the extrudate viathe mandrel to maintain patency of the lumen. Upon take-up, theextrudate is lightly tensioned and cut into sections of predeterminedlength, at which point the extrudate is called a “wet” green tube.

[0058] With respect to the present inventive extruder, concentricalignment of the mandrel is obtainable in both static (i.e., nopressure) and dynamic (i.e., pressure application) portions of anextrusion cycle. Alignment of the mandrel, die, barrel and ram ismaintained by the parallel orientation of the platens that is achievedby countersinking of truss members thereinto and the self-centeringfeature of the mandrel. Thin-walled PTFE tubes are thereby consistentlyderived from the extruder so as to decrease variability in the finalproduct. Such predictable and uniform production further increasesmanufacturing yields and facilitates assembly, maintenance and cleaningof the extruder device between extrusion cycles.

[0059] Various changes and modifications can be made to the presentinvention. It is intended that all such changes and modifications comewithin the scope of the invention as set forth in the following claims.

What is claimed is:
 1. An apparatus for extrusion of at least onethin-walled tubular member from a preform of polymeric material, saidapparatus comprising: an extrudate die having an orifice formedtherethrough; a barrel having a bore in communication with said orifice;a piston slideably disposed in said bore; a ram mounted to said piston;and a guide rod extending through said bore, said guide rod beingconstrained so as to maintain a relatively constant annular gap betweensaid barrel and said guide rod.
 2. An apparatus as in claim 1, whereinsaid guide rod being constrainedly held by at least two locationsspaced-apart along the length of said guide rod.
 3. An apparatus as inclaim 1 further comprising a first platen disposed transversely to saidguide rod, said guide rod being constrainedly held by said first platen.4. An apparatus as in claim 3 further comprising a second platen spacedfrom and arranged generally parallel to said first platen, said ram andsaid guide rod passing through said second platen.
 5. An apparatus as inclaim 4 further comprising a plurality of truss members extendingbetween and connecting said first and second platens.
 6. An apparatus asin claim 5, wherein at least one of said truss members is formed toextend along a longitudinal axis, said truss member having a centralbody portion and a first end portion, said first end portion beingformed cross-sectionally smaller than said central body portion.
 7. Anapparatus as in claim 6, wherein said first platen is formed with atleast one first orifice, said truss member passing through said firstorifice, said first orifice having a first diameter portion registeringwith a first portion of said central body portion, and a reduceddiameter portion registering with said first end portion.
 8. Anapparatus as in claim 7, wherein said first platen is formed with athickness, said first portion of said central body portion having alength of at least 33.3% of said thickness.
 9. An apparatus as in claim7, wherein said truss member is formed with a second end portioncross-sectionally smaller than said central body portion, and whereinsaid second platen is formed with at least one second orifice, saidtruss member passing through said second orifice, said second orificehaving a second diameter portion registering with a second portion ofsaid central body portion, and a second reduced diameter portionregistering with said second end portion.
 10. An apparatus as in claim9, wherein said second platen is formed with a second thickness, saidsecond portion of said central body portion having a length of at least33.3% of said second thickness.
 11. An apparatus as in claim 6, whereinsaid second platen is formed with at least one second orifice, saidtruss member passing through said second orifice, said second orificehaving a second diameter portion registering with a first portion ofsaid central body portion, and a second reduced diameter portionregistering with said first end portion.
 12. An apparatus as in claim11, wherein said second platen is formed with a thickness, said firstportion of said central body portion having a length of at least 33.3%of said thickness.
 13. An apparatus as in claim 5, wherein a traverseplaten is slidably disposed on said trusses, said ram being coupled tosaid traverse platen.
 14. An apparatus as in claim 13 further comprisingdrive pistons coupled to said traverse platen to impart driving forcesthereto.
 15. An apparatus as in claim 14, wherein said drive pistonshave stems coupled to said traverse platen, wherein at least one of saidstems is formed to extend along a longitudinal axis, said stem having acentral body portion and an end portion, said end portion being formedcross-sectionally smaller than said central body portion.
 16. Anapparatus as in claim 15, wherein said traverse platen is formed with atleast one orifice, one of said stems passing through said orifice, saidorifice having a first diameter portion registering with a first portionof said central body portion, and a reduced diameter portion registeringwith said end portion.
 17. An apparatus as in claim 16, wherein saidtraverse platen is formed with a thickness, said first portion of saidcentral body portion having a length of at least 33.3% of saidthickness.
 18. An apparatus as in claim 5 further comprising a frontplaten spaced from and arranged generally parallel to said secondplaten, said extrudate die being mounted to said front platen.
 19. Anapparatus as in claim 18 further comprising a plurality of secondarytruss members extending between and connecting said second platen andsaid front platen.
 20. An apparatus as in claim 19, wherein at least oneof said secondary truss members is formed to extend along a longitudinalaxis, said secondary truss member having a central body portion and afirst end portion, said first end portion being formed cross-sectionallysmaller than said central body portion.
 21. An apparatus as in claim 20,wherein said second platen is formed with at least one second orifice,said secondary truss member passing through said second orifice, saidsecond orifice having a first diameter portion registering with a firstportion of said central body portion, and a reduced diameter portionregistering with said first end portion.
 22. An apparatus as in claim21, wherein said second platen is formed with a thickness, said firstportion of said central body portion having a length of at least 33.3%of said thickness.
 23. An apparatus as in claim 21, wherein saidsecondary truss member is formed with a second end portioncross-sectionally smaller than said central body portion, and whereinsaid front platen is formed with at least one front orifice, saidsecondary truss member passing through said front orifice, said frontorifice having a second diameter portion registering with a secondportion of said central body portion, and a second reduced diameterportion registering with said second end portion.
 24. An apparatus as inclaim 23, wherein said front platen is formed with a thickness, saidsecond portion of said central body portion having a length of at least33.3% of said thickness.
 25. An apparatus as in claim 20, wherein saidfront platen is formed with at least one front orifice, said secondarytruss member passing through said front orifice, said front orificehaving a second diameter portion registering with a first portion ofsaid central body portion, and a reduced diameter portion registeringwith said first end portion.
 26. An apparatus as in claim 25, whereinsaid front platen is formed with a thickness, said first portion of saidcentral body portion having a length of at least 33.3% of saidthickness.
 27. An apparatus as in claim 19, wherein at least a portionof said secondary truss members are located in proximity to said barrel.28. An apparatus as in claim 4, said guide rod being constrainedly heldby said second platen.
 29. An apparatus as in claim 1 further comprisinga wiper element, said guide rod being constrainedly held by said wiperelement.
 30. An apparatus as in claim 1, wherein said guide rod ishollow and coupled to a source of a pressurized stable medium.
 31. Anapparatus as in claim 30, wherein said stable medium is air.
 32. Anapparatus as in claim 1, wherein the tubular member has a wall thicknessof less than or equal to about 200 μm.
 33. An apparatus as in claim 1,wherein the polymeric material is PTFE.
 34. An apparatus as in claim 1,wherein said guide rod is centrally disposed within said barrel.
 35. Anapparatus as in claim 1, wherein said extrudate die is removable.
 36. Anapparatus as in claim 1, wherein said extrudate die is heated.
 37. Anapparatus as in claim 1, wherein said barrel is heated.
 38. An apparatusas in claim 1, further comprising a flexible mandrel connected to saidguide rod.
 39. An apparatus as in claim 38, wherein said mandrel atleast partially extends into said orifice of said extrudate die.
 40. Anapparatus as in claim 38, wherein said mandrel includes first and secondportions, said first and second portions being flexibly connected. 41.An apparatus as in claim 40, wherein said first and second portions areflexibly connected via a ball and socket joint with said first portionbeing formed at one end with a ball-shaped joint portion, saidball-shaped joint portion being receivingly disposed in a socket formedin said second portion.
 42. An apparatus as in claim 40, wherein saidsecond portion is rigidly mountable onto said guide rod.
 43. Anapparatus as in claim 42, wherein said second portion is threadedlymountable onto said guide rod.
 44. An apparatus as in claim 1, furthercomprising a mandrel flexibly connected to said guide rod.
 45. Anapparatus as in claim 44, wherein said mandrel at least partiallyextends into said orifice of said extrudate die.
 46. An apparatus as inclaim 44, wherein said mandrel is partially threaded onto said guiderod.
 47. A thin-walled tubular member extruded by said apparatus ofclaim
 1. 48. A tubular member as in claim 47, wherein said tubularmember has a wall thickness of less than or equal to about 250 μm.
 49. Atubular member as in claim 47, wherein the tubular member is formed ofPTFE.
 50. A process for forming a tube, said process comprising thesteps of: extruding a tubular member having a lumen; and injecting astable medium into said lumen of said tubular member to providestructural support therefor.
 51. A process as in claim 50, wherein saidstable medium is air.
 52. A process as in claim 50, wherein said tubularmember is extruded from a fluoropolymer.
 53. A process as in claim 52,wherein said fluoropolymer is PTFE.
 54. An apparatus for extrusion of atleast one tubular member, said apparatus comprising: an extrudate diehaving an orifice formed therethrough; a barrel having a bore incommunication with said orifice; a guide rod extending through saidbore; and, a flexible mandrel connected to said guide rod, wherein atleast a portion of said mandrel is movable relative to said guide rod.55. An apparatus as in claim 54, wherein said mandrel at least partiallyextends into said orifice of said extrudate die.
 56. An apparatus as inclaim 54, wherein said mandrel includes first and second portions, saidfirst and second portions being flexibly connected.
 57. An apparatus asin claim 56, wherein said first and second portions are flexiblyconnected via a ball and socket joint with said first portion beingformed at one end with a ball-shaped joint portion, said ball-shapedjoint portion being receivingly disposed in a socket formed in saidsecond portion.
 58. An apparatus as in claim 56, wherein said secondportion is rigidly mountable onto said guide rod.
 59. An apparatus as inclaim 58, wherein said second portion is threadedly mountable onto saidguide rod.
 60. An apparatus as in claim 55, wherein said mandrel ismovable relative to said orifice.
 61. An apparatus for extrusion of atleast one tubular member, said apparatus comprising: an extrudate diehaving an orifice formed therethrough; a barrel having a bore incommunication with said orifice; a guide rod extending through saidbore; and, a mandrel flexibly connected to said guide rod such that saidmandrel is movable relative to said guide rod.
 62. An apparatus as inclaim 61, wherein said mandrel at least partially extends into saidorifice of said extrudate die.
 63. An apparatus as in claim 61, whereinsaid mandrel is partially threaded onto said guide rod.
 64. An apparatusas in claim 62, wherein said mandrel is movable relative to saidorifice.